Diamond crystals are typically used for the production of diamond tools, such as grinding wheels, dressing or truing tools for grinding wheels, and saw blades.
To function effectively in the above-mentioned applications, diamond crystals having the highest strength, including compressive fracture strength, are desirable. The compressive fracture strength is a key specification for diamond crystals, and is often correlated with the diamond's performance in such applications. Crystal fracture strength, as measured by Roll Crusher, correlates with performance of diamond in these applications, such as stone cutting.
The compressive fracture strength may be measured by compressively fracturing a population of diamond grains both before and after treatment. The Roll Crusher method utilizes an apparatus having a pair of hard counter-rotating rollers adapted with a means to measure the compressive force applied by the rollers to grains passing between the rollers at the moment of grain fracture. The motion of one of the rollers is measured by a suitable transducer, such as a linear voltage differential transformer, to generate an electric signal proportional in voltage to the deflection of the roller, and hence, proportional to the compressive force on the diamond grain.
Diamond is a brittle solid and fails by fracture. Its elastic constant (Young's modulus of diamond) is high: 1.143×1012 pascals. The ductile-brittle transition temperature for diamond is about 1150° C. The above applications use diamond in the brittle fracture region extending from room temperature to about 1000 ° C.
Impurity atoms dissolved in the diamond crystal, such as boron, nitrogen, and hydrogen, can generate stresses because of the large lattice dilations that they cause. For example, dissolved nitrogen, boron, and hydrogen atoms expand the diamond lattice around them by 40%, 33.7%, and 31%, respectively. If the distribution of dissolved nitrogen, boron, or hydrogen is uniform in a diamond crystal, the lattice just expands uniformly, and no long-range stresses develop. However, if the distribution of nitrogen, boron, or hydrogen is not uniform, inhomogeneous strains will occur in the diamond. These uneven strains will generate large long-range stresses.
Presently, the impurity concentration in synthetic diamond crystals decreases with increasing radius within the diamond crystal. There are several reason for this. For instance, in the High Pressure High Temperature (HPHT) process used to synthesize diamond crystals, the concentration of nitrogen in the melt decreases with time because the growing diamond takes up nitrogen from the melt. (“Melt” refers to the molten, metallic catalyst/solvent through which carbon is transported from the graphite feedstock to the growing diamond crystals at high pressure and high temperature.). Another reason is that the growth rate of the diamond crystal is decreasing with increasing radius, and the impurity incorporation decreases with decreasing growth rate. Further, the impurity concentration in the diamond crystal may depend on the growth sector. By “growth sector” is meant the crystallographic direction in which growth took place (or is taking place), and the region(s) in the crystal in which growth occurred in the same direction.
This decreasing impurity concentration with increasing radius (i.e., negative concentration gradient) causes tangential tensile stresses on the surface of the diamond. Since diamond is a brittle solid, its compressive fracture strength is reduced by these tangential tensile stresses.
As technology advances, the next generation of diamond will require higher strength. Thus, there is a need for a diamond crystal with increased compressive fracture strength. There is also a need for methods to manufacture these diamond crystals.